A-site cation–pseudohalide co-design for tin halide perovskite transistors

Tin halide perovskites are promising lead-free semiconductors for next-generation electronics; however, their implementation is constrained by a fragile crystal framework, facile Sn 2+ oxidation, and rapid crystallization kinetics. Here, we establish a molecular design principle for non-metallic thiocyanate (SCN − ) additives that stabilizes the formamidinium tin iodide (FASnI 3 ) lattice and enables high-performance, stable p-channel thin-film transistors (TFTs). Density functional theory calculations reveal that the Sn–X–Sn bond strength in the lattice and the halide migration barriers can be tuned by the size of the A-site cation and its hydrogen-bonding capability with SCN − and I − . In particular, we find that both the Sn–SCN–Sn bonding interaction and the iodine migration barrier are maximized when guanidinium thiocyanate (GuaSCN) is employed. Consistently, GuaSCN enhances the crystallinity and grain size of tens-of-nanometer-thick FASnI 3 films, prolongs carrier lifetimes, and suppresses Sn 4+ formation, without altering the intrinsic bandgap. TFTs incorporating GuaSCN achieve a field-effect mobility above 88 cm 2 V −1 s −1 , an I on/off ratio exceeding 4 × 10 9 , negligible hysteresis, and superior operational and shelf stability across hundreds of devices.